Peroxymonosulfuric acid (Caro's acid) and its salts (peroxymonosulfates) are becoming increasingly important as oxidation agents in the oxidative removal of pollutants from waste waters. Caro's acid is effective in such methods; however, it must be produced in situ or directly prior to being used because of its limited storage stability. The etching solutions containing Caro's acid for the production of printed circuits such as those described e.g. in European Patent Application EP-A 0 256 284 have limited stability as regards the available oxygen; under the indicated test conditions -- 5 hours at 70.degree. C. -- the loss is approximately 15% if the solution used contains approximately 4.6% by weight peroxymonosulfuric acid. Thus, such solutions can not be marketed.
Even though peroxymonosulfates of the alkaline metals and alkaline-earth metals as well as of a few other metals and of ammonia are known -- cf. German patent DE 10 80 083 --, only potassium peroxymonosulfate in the form of the storage-stable triple salt 2KHSO.sub.5 KHSO.sub.4 K.sub.2 SO.sub.4 and ammonium peroxymonosulfate in the form of a solution, the so-called "ammonium-stabilized Caro's acid", have become accepted in practice. However, since the above-mentioned potassium triple salt must generally be dissolved prior to use and the ammonium ion of the Caro's acid stabilized therewith must be considered itself as a critical component of waste water, there is a need for readily manageable and ecotoxicologically harmless, storage-stable peroxymonosulfate solutions. Absolute eco-toxicological safety could be expected from solutions containing sodium peroxymonosulfate.
According to the methods described in DE patent 10 80 083, metal monopersulfates are prepared by means of reacting Caro's acid or mixtures of Caro's acid and sulfuric acid with the hydroxides or carbonates of the metals concerned up to a pH of the reaction mixture of not more than 3. The reaction mixture which is obtained at first is transferred immediately after its preparation by means of vacuum drying or spray [flash]drying into a salt mixture containing the corresponding peroxymonosulfate. In order to obtain sufficiently storage-stable products, diluting agents with a stabilizing action are added preferably before or during the drying.
Solutions containing Caro's acid and sulfuric acid can be used in the method of DE 10 80 083 which were obtained either by means of electrolysis of a sulfuric acid with a content of preferably 35% to 50% by weight, with subsequent hydrolysis of the peroxodisulfuric acid formed or by means of the reaction of 50% to 88% by weight aqueous hydrogen peroxide with concentrated or fuming sulfuric acid (oleum). In order to prepare sodium peroxymonosulfate, such H.sub.2 SO.sub.5 /H.sub.2 SO.sub.4 solutions are neutralized with a soda solution up to pH 2.9, freed of the precipitated Glauber salt and subsequently dried.
The method evaluated above for the preparation of solutions containing NaHSO.sub.5 proved to be unsatisfactory because a considerable loss of available oxygen was observed during the neutralization -- even in the relative example of DE 10 80 083, the yield of NaHSO.sub.5 in the solution is indicated at only 85% of theory. In addition, the storage stability of the solution was unsatisfactory: The available oxygen content decreased to an intolerable degree and at the same time the solution was opacified again and again by precipitating Glauber salt. Obviously, the problems detected during the reworking of the method were also the cause for the fact that the usefulness of the solution was not pointed out anywhere in DE 10 80 083, but rather it was always concentrated by evaporation. However, the drying of the solution containing NaHSO.sub.5 in order to obtain a storage-stable, solid product is associated with a further loss of available oxygen and is also expensive, which reduces the economic value of the process.